Maleic anhydride has been produced by the vapor phase oxidation of n-butane, n-butenes, 1,3-butadiene or mixture thereof in the presence of an oxidation catalyst. It is desired that the catalysts exhibit effective activity and high selectivity toward maleic anhydride, and various catalysts have been proposed which are based on different combinations of several components. However, those catalysts which tend to give the more desirable results generally require components which are relatively expensive, such as vanadium. For example, French Pat. No. 2287-504 to UBE Industries discloses the preparation of maleic anhydride by the catalytic vapor phase oxidation of unsaturated hydrocarbons having 4-6 carbon atoms in the presence of a catalyst containing VP.sub.a Ti.sub.b X.sub.c O.sub.d, wherein X is at least one of Na, Ca, Mg, Fe, Zr, B, Mn, Ag or Mo; a is 1.0 to 5.0 (preferably 2-4); b is 2.0 to 12 (preferably 4.5-10); c is 0 to 1; and d satisfies the valence of the other elements present (preferably 8-40). Belgium Pat. No. 821-051 to BASF discloses the preparation of maleic anhydride by the catalytic vapor phase oxidation of linear unsaturated aliphatic hydrocarbons having 4 or more carbon atoms in the presence of a supported catalyst containing 2-25% of V.sub.2 O.sub.5 (preferably 2-10)); 1-35% of P.sub.2 O.sub.5 (preferably 3-25%); and 40-97% of TiO.sub.2 (preferably 65-95%), the active catalytic material being 50-1500% of the support (preferably 100-600%).
British Pat. No. 1,157,117 discloses the production of maleic anhydride from a saturated aliphatic hydrocarbon having 4 carbon atoms or an unsaturated aliphatic hydrocarbon having 4 or 5 carbon atoms in the presence of a catalyst comprising an oxide of molybdenum and at least one other oxide of tin, antimony, titanium, iron or tungsten. This catalyst may optionally contain an acidic oxide of phosphorus or boron.
By the process of the present invention, the stability and activity of the catalyst are improved, and unlike most processes that involve the catalytic vapor phase oxidation of 4-C hydrocarbons to produce maleic anhydride, the catalyst composition does not require vanadium as an essential element.